Turbofan gas turbine engines (which may be referred to simply as ‘turbofans’) are typically employed to power aircraft. Turbofans are particularly useful on commercial aircraft where fuel consumption is a primary concern. Typically a turbofan gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven directly off an additional lower pressure turbine in the engine core.
The fan comprises an array of radially extending fan blades mounted on a rotor and will usually provide, in current high bypass gas turbine engines, around seventy-five percent of the overall thrust generated by the gas turbine engine. The remaining portion of air from the fan is ingested by the engine core and is further compressed, combusted, accelerated and exhausted through a nozzle. The engine core exhaust mixes with the remaining portion of relatively high-volume, low-velocity air bypassing the engine core through a bypass duct.
To satisfy regulatory requirements, such engines are required to demonstrate that if part or all of a fan blade were to become detached from the remainder of the fan, that the detached parts are suitably captured within the engine containment system.
The fan is radially surrounded by a fan casing. It is known to provide the fan casing with a fan track liner and a containment system designed to contain any released blades or associated debris. Often, the fan track liner can form part of the fan containment system.
The fan track liner typically includes an annular layer of abradable material which surrounds the fan blades. During operation of the engine, the fan blades rotate freely within the fan track liner. At their maximum extension of movement and/or creep, or during an extreme event, the blades may cut a path into this abradable layer creating a seal against the fan casing and minimising air leakage around the blade tips.
An operational requirement of the fan track liner is that it is resistant to ice impact loads. A rearward portion of the fan track liner is conventionally provided with an annular ice impact panel. This is typically a glass-reinforced plastic (GRP) moulding which may also be wrapped with GRP to increase its impact strength. Ice which forms on the fan blades is acted on by both centrifugal and airflow forces, which respectively cause it to move outwards and rearwards before being shed from the blades.
The geometry of a conventional fan blade is such that ice is shed from a trailing edge of the blade, strikes the ice impact panel, and is deflected without damaging the panel.
Swept fan blades are increasingly used in turbofan engines as they offer significant advantages in efficiency over conventional fan blades. Swept fan blades have a greater chord length at their central portion than conventional fan blades. This greater chord length means that ice that forms on a swept fan blade follows the same rearward and outward path as on a conventional fan blade but may reach a radially outer tip of the blade before it reaches the trailing edge. The ice will therefore be shed from the blade tip and may strike the fan track liner forward of the ice impact panel within the blade off zone (that is the region where a blade would contact the fan track liner in the event of a blade being detached from the fan).
A fan track liner used with a swept fan blade is therefore required to be strong enough to resist ice impact whilst allowing a detached fan blade to penetrate and be contained therewithin.
In recent years there has been a trend towards the use of lighter fan blades, which are typically either of hollow metal or of composite construction. These lighter fan blades have similar impact energy per unit area as an ice sheet, which makes it more difficult to devise a casing arrangement that will resist the passage of ice and yet not interfere with the trajectory of a released fan blade.
The fan casing is typically a plain or ribbed metallic casing, or a plain or isogrid Kevler® (a para-aramid synthetic fibrous material from DuPont™) wrapped casing. In order to absorb the high energies generated following the detachment of a fan blade, the material(s) used to form the casing are selected for high strength and high ductility.
Early containment systems incorporated a steel band wrapped around the casing in the plane of the rotating fan blade. To reduce weight, a Kevlar® wrapped aluminium fan case was introduced. During fan blade off, the Kevlar® absorbs the blade energy by deflecting and stretching, to distribute the load around the casing, and in some designs of soft wall casing, the blade penetrates the case and is caught and contained by the Kevlar® wrap. Further alternative casing arrangements are available, including ribbed Armco® (a ferrous material from AK Steel International Ltd) or ribbed titanium, which are more usually used in hard wall designs where the blade is designed not to penetrate the case.
A conventional hard wall fan containment system or arrangement 100 is illustrated in FIG. 1 and surrounds a fan comprising an array of radially extending fan blades 40. Each fan blade 40 has a leading edge 44, a trailing edge 45 and fan blade tip 42. The fan containment arrangement 100 comprises a fan case 150. The fan case 150 has a generally frustoconical or cylindrical annular casing element 152 and a hook 154. The hook 154 is positioned axially forward of an array of radially extending fan blades 40. A fan track liner 156 is mechanically fixed or directly bonded to the fan case 150. The fan track liner 156 may be adhesively bonded to the fan case 150. The fan track liner 156 is provided as a structural filler to bridge a deliberate gap provided between the fan case 150 and the fan blade tip 42.
The fan track liner 156 has, in circumferential layers, an attrition liner 158 (also referred to as an abradable liner or an abradable layer), a filler layer which in this embodiment is a honeycomb layer 160, and a septum 162. The septum layer 162 acts as a bonding, separation, and load spreading layer between the attrition liner 158 and the honeycomb layer 160. The honeycomb layer 160 may be an aluminium honeycomb. The tips 42 of the fan blades 40 are intended to pass as close as possible to the attrition liner 158 when rotating. The attrition liner 158 is therefore designed to be abraded away by the fan blade tips 42 during abnormal operational movements of the fan blade 40 and to just touch during the extreme of normal operation to ensure the gap between the rotating fan blade tips 42 and the fan track liner 156 is as small as possible without wearing a trench in the attrition liner 158. During normal operations of the gas turbine engine, ordinary and expected movements of the fan blade 40 rotational envelope cause abrasion of the attrition liner 158. This allows the best possible seal between the fan blades 40 and the fan track liner 156 and so improves the effectiveness of the fan in driving air through the engine.
The purpose of the hook 154 is to ensure that, in the event that a fan blade 40 detaches from the rotor of the fan 12, the fan blade 40 will not be ejected through the front, or intake, of the gas turbine engine. During such a fan-blade-off event, the fan blade 40 travels tangentially to the curve of rotation defined by the attached fan blades. Impact with the containment system (including the fan track liner 156) of the fan case 150 prevents the fan blade 40 from travelling any further outside of the curve of rotation defined by the attached fan blades. The fan blade 40 will also move forwards in an axial direction, and the leading edge 44 of the fan blade 40 collides with the hook 154. Thus the fan blade 40 is held by the hook 154 and further axially forward movement is prevented. A trailing blade (not shown) then forces the held released blade rearwards where the released blade is contained. Thus the fan blade 40 is unable to cause damage to structures outside of the gas turbine engine casings.
As can be seen from FIG. 1, for the hook 154 to function effectively, a released fan blade 40 must penetrate the attrition liner 158 in order for its forward trajectory to intercept with the hook. If the attrition liner 158 is too hard then the released fan blade 40 may not sufficiently crush the fan track liner 156.
However, the fan track liner 156 must also be stiff enough to withstand the rigours of normal operation without sustaining damage. This means the fan track liner 156 must be strong enough to withstand ice and other foreign object impacts without exhibiting damage for example. Thus there is a design conflict, where on one hand the fan track liner 156 must be hard enough to remain undamaged during normal operation, for example when subjected to ice impacts, and on the other hand allow the tip 42 of the fan blade 40 to penetrate the attrition liner 158. It is a problem of balance in making the fan track liner 156 sufficiently hard enough to sustain foreign object impact, whilst at the same time, not be so hard as to alter the preferred hook-interception trajectory of a fan blade 40 released from the rotor. Ice that impacts the fan casing rearwards of the blade position is resisted by an ice impact panel 164.
An alternative fan containment system is indicated generally at 200 in FIG. 2. The fan containment system 200 includes a fan track liner 256 that is connected to the fan casing 250 at both an axially forward position and an axially rearward position. At the axially forward position, the fan track liner is connected to the casing at hook 254 via a sprung fastener 266. In the event of a fan blade detaching from the remainder of the fan, the fan blade impacts the fan track liner 256 and the fan track liner pivots about the rearward position of attachment to the casing (indicated at 268 in FIG. 2). Such an arrangement is often referred to as a trap door arrangement. The trap door arrangement has been found to help balance the requirements for stiffness of the fan track liner with the requirements for resistance of operational impacts (e.g. ice impacts) ensuring a detached blade is held within the engine.
The fan track liner may be formed of a plurality of arcuate panels positioned substantially coaxially so as to form a cylindrical or frustoconical fan track liner. A fan track liner panel of the prior art is indicated generally at 370 in FIG. 3. The fan track liner panel 370 includes straight edges 372a, 372b in the axial direction.
When the fan containment system has a trap door arrangement, the trajectory of a released fan blade or a released part of a fan blade (reference to a released fan blade from hereon in refers to both a released fan blade and a released part of a fan blade) can cross the boundary from one fan track liner panel to another. When a fan blade is released the trap door of a first fan track liner panel will be activated. However, the trap door of adjacent fan track liner panels will remain closed unless a sufficient force is applied to open them. This means that a step is present between the fan track liner panel where the trap door has been activated and the fan track liner panel where the trap door has not yet been activated. The step creates a barrier to a released fan blade, so there is a concern that the released fan blade may skip over the hook and avoid containment.
A contemplated solution to this problem is to adhesively bond adjacent panels together. However, the use of adhesive creates problems for both assembly and on-wing repair. An advantage of providing a fan track liner made from a plurality of panels is that liner damage can be quickly and effectively addressed whilst the engine is on-wing with minimum disruption. If an adhesive is used this advantage is reduced because of the need to remove adhesive from the panels and wait for adhesive to cure once repair work is complete.